A detailed evaluation of the performance of a hexapole collision and reaction cell in inductively coupled plasma mass spectrometry (ICP-MS) for the speciation of selenium in human urine has been carried out. The relative limits of detection, monitoring isotope ⁸⁰Se, were in the range 30â 80â ppt (using a hydraulic high pressure nebulizer) and 100â 200â ppt (using a Meinhard nebulizer) for the seleno-compounds not interfered by the urinary matrix. Selenium species separation has been carried out by reversed-phase and ion-pair high-performance liquid chromatography (HPLC). Also, the chromatographic behaviour of some sulfur compounds was studied. Unfortunately, the retention times of the selenoaminoacids and the analogue sulfur compounds were not the same, and some chemical interactions between selenite and cysteine or reduced glutathion were observed. The urinary selenium speciation was carried out in 8 different subjects before and after 10 days of supplementation with different commercial nutritional tablets. Trimethylselenonium was found in 3 subjects as one of the main selenium species in urine before and after supplementation. However, a compound, not identified yet, was observed to be the main urinary selenium species after supplementation in all cases.